Fluid mixed with plural solid components, such as river water, sea water, or blood, are known. This fluid is mixture of liquid component and solid components. The solid components, such as sand, bacteria, or blood corpuscle cells, exist as solid substances by precipitation or dispersion while not being dissolved in the liquid component.
As a method of separating the plural components, a device for separating blood corpuscle from blood will be described. Blood is generally sampled as whole blood including blood plasma as the liquid component, blood corpuscle cells as the solid components, and other components. However, either only the blood corpuscle cell or only the blood plasma is often required for examining the blood.
For example, in order to examine a blood sugar level in blood, it is necessary to measure blood sugar dissolved in a blood plasma. In order to detect DNA, DNA is taken out from a white blood corpuscle cell in blood corpuscle. Therefore, in order to separate blood plasma and blood corpuscle components from whole blood sampled by a conventional method, the whole blood is put in a test tube, and a predetermined centrifugal force is applied to it with a centrifugal machine. The components in the whole blood in the test tube receive respective centrifugal forces corresponding to the components, thereby being separated by the difference of their masses.
Then, the blood plasma component is obtained by extracting supernatant fluid, and the blood corpuscle component is obtained from precipitate. Then, the separated components are subjected to predetermined measurements in an examination process.
The conventional method using the centrifugal machine has the following problem. This method requires a certain amount of the sample, for example, several milliliters to several tens milliliters of the whole blood in the test tube. By this method, it is difficult to separate the liquid component or the solid component from a small amount of the sample.
A method of separating the solid component from a small amount of sample using a filter is disclosed in “Integrated vertical screen micro-filter system using inclined SU-8 structure” (Yong-Kyu Yoon, MEMS2003, Kyoto, pp. 227-230 published by IEEE). A porous filter filters blood corpuscles having sizes larger than a predetermined size, thereby separating blood corpuscles from a blood plasma component. In this method, the size and the number of pores of the filter affect its separation characteristics. Therefore, the filter is designed optimally to the component to be separated. For example, a meshed filter can be manufactured precisely regarding the size and the number of the pores of the filter by exposing a light sensitive resist three-dimensionally.
In this conventional method employing the filter, the particle size of the solid component passing through the filter depends on a pressure applied to fluid or powder fluid containing plural components mixed therein passing through the filter. The solid component particles, particularly having plural sizes, are hardly separated by this method. In order to take out the predetermined particles, the sizes of the pores of the filters are determined to allow particles smaller than the predetermined particles to pass through the pores. The predetermined particles are trapped by the filter, and thus, may clog the pores of the filter, thereby preventing small particles from passing through.
Japanese Patent Laid-Open Publication No. 2001-525722 discloses a device for performing manipulation of particles suspended in fluid. This device includes a duct for allowing a fluid containing particles suspended therein to flow, an ultrasonic transducer provided at a side of the duct, and a reflector provided at another side of the duct. The ultrasonic transducer is provided on a side surface of a flow path of the duct. The ultrasonic transducer contacts the inside of the duct. In this device, an acoustic standing wave which passes across the duct in its width direction is generated, and thereby, condenses the particles to form one or more planer bands extending in parallel with the vertical axis of the duct, thereby separating the particles, i.e., solid components, from liquid component.
In this device, the acoustic standing wave allows the particles to concentrate at predetermined positions in the fluid, and it is not necessary to be anxious about clogging of a filter. However, since contacting the inside of the duct, the ultrasonic transducer may be contaminated by the fluid in the duct. Since the ultrasonic transducer forms a part of the duct, the duct is necessarily formed of flat surfaces, hence causing a vibration surface of the ultrasonic transducer to be a plane surface. Therefore, acoustic waves generated by the transducer are limited to plane waves. It is difficult to install an ultrasonic actuator precisely to the side surface of the duct.